Semiconductor memory devices, which may be used to store various types of data, can be divided into volatile memory devices and non-volatile memory devices. The volatile memory devices are typically represented by a dynamic random access memory (DRAM) and a static random access memory (SRAM). The DRAM stores data by using electric charges stored in a capacitor. To prevent the electric charges from leaking off the capacitor, a refresh operation is necessary. However, a power supply is stopped, the data stored in the DRAM may disappear. On the contrary, the non-volatile memory devices keep the stored data even when power is terminated. For this reason, non-volatile memory devices are used widely in circumstances where power cannot be supplied continuously, such as mobile phones and diverse application devices including a memory card for storing music or video data. The non-volatile memory devices include a flash memory representing ‘0’ and ‘1’ of data based on whether tunneling charges are stored or not, a Ferroelectric Random Access Memory (FRAM) using the polarization direction of a dielectric substance, a magnetic RAM (MRAM) using the magnetization direction of a magnetic substance. The flash memory has a disadvantage of slow data erase/write speed and the FRAM has a disadvantage of supporting relatively few rewrite operations (i.e., low reusable number).
The magnetic RAM, which typically does not have the above disadvantages has been attracting attention recently. The MRAM has advantages that it does not have a limit in the reusable number, and it can be highly integrated. Also, it can be operated at a high speed. In a magnetic memory device, data is stored in a simple thin ferromagnetic film or in a multi-layer magnetic thin film, (e.g., Tunneling Magneto-Resistance (TMR)), or in a Giant Magneto-Resistance (GMR). The basic structure and operation of a conventional magnetic memory device using the TMR will be described hereinafter with reference to the drawings.
FIG. 1 is a plane view showing a conventional magnetic memory device, and FIGS. 2A and 2B are cross-sectional views obtained by cutting the magnetic memory device of FIG. 1 along line 2A-2A′ and line 2B-2B′, respectively. Referring to FIG. 1, a plurality of conducting wires are formed perpendicularly to each other on a semiconductor substrate to thereby form bit lines 30 and digit lines 28. Magnetic storage elements 40 for storing data are formed in the area where the bit lines 30 and the digit lines 28 cross each other. Referring to FIGS. 2A and 2B, an field isolation layer 12 is positioned in a predetermined area of the semiconductor substrate 10 to define active regions. In each active region, a pair of gate electrodes 20 are formed. The gate electrodes 20 include a gate of a transistor used to read data stored in the magnetic storage elements 40. A common source region 16s is formed between the gate electrodes 20, and a drain region 16d is formed between a gate electrode 20 and an field isolation layer 12. The common source region 16s is connected to a common source electrode 18, and the drain region 16d is connected to a vertical wire 24. A bottom interlayer insulation layer 22 is formed on the entire surface of the semiconductor substrate 10 including the digit lines 28. The vertical wire 24 electrically connects the drain region 16d to a bottom electrode 26, which is formed in the upper part of the bottom interlayer insulation layer 22 with a space therefrom. The bottom electrode 26 is connected to the magnetic storage element 40, and a bit line 30 is formed on top of the magnetic storage element 40. Herein, the bottom electrode 26 and the magnetic storage element 40 is insulated by a top interlayer insulation layer 32 formed on top of the bottom interlayer insulation layer 22.
The magnetic storage element 40, which has a structure of Magnetic Tunnel Junction (MTJ) includes a pinning layer 41, a fixed layer 42, an insulating layer 43, and a free layer 44. The resistance of the magnetic storage element 40 is varied according to whether the magnetization directions of the free layer 44 and the fixed layer 42 are the same or not. The resistance characteristic of the magnetic storage element 40, which is dependent on the magnetization direction, is utilized as a data storing mechanism of the magnetic memory device. The magnetization direction of the fixed layer 42 is not changed during a typical reading/writing operation, and the pinning layer 41 fixes the magnetization direction of the fixed layer 42. On the contrary, the magnetization direction of the free layer 44 is variable. The free layer 44 can be magnetized in the same direction as the fixed layer 42 or can be magnetized in the opposite direction.
When data stored in a particular magnetic storage element 40 is read, the bit lines 30 and word lines 20 are used. The word lines 20 correspond to the gate electrodes 20 formed on the semiconductor substrate 10 and they are formed in perpendicular to the bit lines 30. When electric current flows into the magnetic storage element 40 by selecting a word line 20 and a bit line 30, the amplitude of the electric current is different according to the data storage state. In other words, the stored data can be read, because the resistance value is different according to whether the magnetization directions of the fixed layer 42 and the free layer 44 are the same and the amplitude of the electric current is different according to the resistance value. Meanwhile, data are recorded by providing electric current to a bit line 30 and a digit line 28 to select a particular magnetic storage element 40 and magnetizing the selected magnetic storage element 40 based on a vector addition of a magnetic field formed by the electric current.
Hereafter, problems of the conventional magnetic memory device will be described. When electric current flows through the bit lines 30 and the digit lines 28, a magnetic field is formed around the lines 28 and 30. Basically, the magnetic field should affect only the magnetic storage element 40 whose magnetization should be changed. However, as the magnetic memory device is highly integrated, memory cells become close to each other, and the magnitude of the magnetic field needed for writing data increases as well. Therefore, there can be a problem that the magnetic field generated by the bit lines 28 and the digit lines 30 affects not only the selected magnetic storage element 40 but also an adjacent magnetic storage element 40. To solve this problem, a magnetic memory device having a new structure not using the bit lines 28 is suggested. According to the suggested technology, the word lines 20 replace the digit lines 28 and the electric current flows through the word lines 20 and the bit lines 30 to thereby form a magnetic field. The formed magnetic field changes the magnetization direction of the magnetic storage element 40. Meanwhile, it is also possible to change the magnetization direction of the magnetic storage element 40 by increasing the amplitude of electric current, which is provided to the magnetic storage element 40 through the word lines 20 and the bit lines 30, more than when data are read while providing the electric current to the magnetic storage element 40 in the same method as data are read. This magnetic memory device is disclosed in U.S. Pat. No. 5,695,864.
The technology that does not use the digit lines 28, which is described above, can prevent other magnetic storage elements 40 from being affected by the magnetic field due to disturbance caused by the digit lines 28. However, it still has a problem of disturbance caused by the bit lines 30. In particular, in the method of providing electric current to the magnetic storage element 40 to record data, since a high amplitude of electric current flows during the data recording, a strong magnetic field is formed and this enhances the possibility that the magnetic field affects adjacent areas. Also, since the method of recording data only by using the bit lines 30 without the digit lines 28 requires a strong magnetic field necessarily, a method for reducing power consumption is needed.